Metal refinement and metal composite materials using carbon nanotubes

ABSTRACT

The present invention relates to metals and matrix composites and methods of manufacture. A method of refining a metal oxide is shown. A method of forming a carbon nanotube metal matrix composite is shown using a metal oxide and an amount of carbon nanotubes. Processing is carried out using wave energy radiation such as microwave radiation. In some methods, processing is carried out while substantially isolated from external oxygen sources such as ambient air.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with U.S. Government support under Grant No. NSF-DGE0751279 awarded by the National Science Foundation. The U.S. Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The discovery of carbon nanotubes (CNTs) has sparked a large amount of innovative research as the unique properties of CNTs and their possible applications are explored. CNT's possess a number of exciting attributes including, but not limited to, high mechanical strength, high thermal conductivity, high electrical conductivity, and light weight.

One application that has been explored is lightweight composites using CNTs as reinforcement. To date, the vast majority of CNT-filled composites employ a polymer matrix. However, the same advantages CNTs afford polymer matrix composites (PMCs) apply to metal matrix composites (MMCs) as well. In particular, CNTs exhibit higher strength, stiffness, thermal conductivity, and electrical conductivity than the matrix phase while having lower density. One reason CNT-MMCs are not as well established as PMCs is the fact that metals are more difficult to process than polymers, because high temperatures and/or large forces are usually needed to introduce reinforcement into the metal matrix.

A number of possible application also exist that do not rely on mechanical strength from CNTs. Innovative methods are desired that exploit the unique properties of CNTs.

SUMMARY

A metal matrix composite is shown, including a reduced metal oxide matrix, and a carbon nanotube dispersed phase. A method of making a material, such as a metal matrix composite is shown, including combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material, heating the starting material using wave energy radiation, and reducing the metal oxide to form a metal. Another method is shown, including combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material, substantially isolating the starting material from external oxygen, heating the starting material using microwave radiation, and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes. Another method is shown, including combining an amount of carbon nanotubes with an amount of a powdered metal oxide, mixing the amount of carbon nanotubes with the amount of powdered metal oxide to form a substantially homogenous starting material, substantially isolating the starting material from external oxygen, heating the starting material using microwave radiation, and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.

Methods of forming materials, and the materials created using these methods may have a number of advantages, including but not limited to low manufacturing cost, high hardness, high strength to weight ratio, improved carbon nanotube integrity after processing, and improved dispersion of the dispersed carbon nanotube phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a materials processing system according to an embodiment of the invention.

FIG. 2 is a flow diagram of an example method according to an embodiment of the invention.

FIG. 3 is x-ray diffraction data of materials according to an embodiment of the invention.

FIG. 4 is Raman spectroscopy data of materials according to an embodiment of the invention.

FIG. 5 is a collection of photomicrographs of materials according to an embodiment of the invention.

FIG. 6 is a collection of electron spectroscopy images of materials according to an embodiment of the invention.

FIG. 7 is a collection of additional electron spectroscopy images of materials according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to certain embodiments of the disclosed subject matter, examples of which are illustrated in part in the accompanying drawings. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter.

Values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a range of “about 0.1% to about 5%” or “about 0.1% to 5%” should be interpreted to include not just about 0.1% to about 5%, but also the individual values (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within the indicated range. The statement “about X to Y” has the same meaning as “about X to about Y,” unless indicated otherwise. Likewise, the statement “about X, Y, or about Z” has the same meaning as “about X, about Y, or about Z,” unless indicated otherwise.

In this document, the terms “a,” “an,” or “the” are used to include one or more than one unless the context clearly dictates otherwise. The term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Any use of section headings is intended to aid reading of the document and is not to be interpreted as limiting; information that is relevant to a section heading may occur within or outside of that particular section. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

In the methods of manufacturing described herein, the steps can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Furthermore, specified steps can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed step of doing X and a claimed step of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.

The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range.

The term “substantially” as used herein refers to a majority of, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more.

FIG. 1 shows a system 100 adapted to form a metal and/or a metal matrix material. FIG. 1 includes a source of wave energy 102. In the example shown, the source of wave energy 102 is a microwave energy source, such as a microwave oven. In one example, microwave energy radiation includes a frequency range between approximately 300 MHz and approximately 300 GHz. Although microwave energy is used as an example, energy of other frequencies are within the scope of the invention. A container 120 is shown within the source of wave energy 102. In one example, the container 120 includes a refractory container such as a crucible. Materials such as refractory metals, or other materials having a melting temperature sufficiently high to resist melting or reaction with a workpiece material are also within the scope of the invention.

A workpiece 122 is shown located within the container 120. In one example, the workpiece 122 includes an amount of carbon particles and an amount of metal oxide particles. In one example, the amount of metal oxide particles includes copper oxide. In one example, the amount of metal oxide particles includes nickel oxide. Other metal oxides such as rare earth oxides, or other metal oxides that may be reduced in a carbothermic reaction are within the scope of the invention. Examples of rare earth oxides that can be reduced to form metals include oxides of Nd, Pr, La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y. In one example, the amount of carbon particles includes an amount of carbon nanotubes. In one example, the amount of carbon nanotubes includes multi-walled carbon nanotubes.

A containment shroud 110 is shown within the source of wave energy 102, substantially isolating the container 120 and workpiece 122 from surrounding atmosphere. In one example, a passage 112 is coupled to the containment shroud 110 within the source of wave energy 102. In one example, the passage 112 is coupled to a gas source that does not contain oxygen. In one example, the passage 112 is coupled to a source of inert gas, such as argon, helium, nitrogen, etc. In one example, the passage 112 is coupled to a vacuum source that is adapted to remove oxygen from within the containment shroud 110.

FIG. 2 shows an example method of forming a metal matrix composite material according to an embodiment of the invention. In operation 200, an amount of carbon nanotubes are combined with an amount of a metal oxide to form a starting material. In one example, the amount of metal oxide includes powdered metal oxide. A number of particle sizes and size distributions may be used within the scope of embodiments of the invention. In one example the metal oxide particles are between approximately 150 mesh and 250 mesh. In one example, more than one particle size is used to increase surface area contact between the amount of carbon nanotubes and the metal oxide.

In operation 202, the amount of carbon nanotubes is mixed with the amount of powdered metal oxide to form a substantially homogenous starting material. Mixing the components of the starting material may more uniformly distribute the carbon nanotubes within the metal oxide. Mixing the components of the starting material may also increase the surface area of contact between the carbon nanotubes and the metal oxide particles.

In one example, a carbothermic reaction that reduces the metal oxide particles and converts them to substantially pure metal is controlled by solid state diffusion at contact regions between the carbon nanotubes and the metal oxide particles. For example, in configurations where external oxygen is substantially removed from the system, an amount of oxygen used in the carbothermic reaction is provided from oxygen in the metal oxide diffusing from a center of metal oxide particles to an interface with carbon nanotubes. In such a configuration, an increased surface area contact between the carbon nanotubes and the metal oxide particles is desired. In one example, the starting material is compacted prior to heating to further increase surface area contact between the carbon nanotubes and the metal oxide particles

In operation 204, the starting material is substantially isolated from external oxygen. In one example, when external oxygen from the atmosphere is used in the carbothermic reaction, a large percentage of the carbon nanotubes may be consumed in the reaction and/or the integrity of the carbon nanotubes may be altered such that the mechanical properties of the resulting metal matrix composite are lower than desired. As discussed above, a number of possible mechanisms may be used to isolate the starting material from external oxygen, including but not limited to immersion in a gas flow that does not include oxygen, such as an inert gas. Other mechanisms may include isolating the starting material in a vacuum.

In one example, the starting material is exposed to air. In selected examples, exposure to air and consumption of the carbon nanotubes is desired. In one example a refined metal is desired, with removal of metal oxide. In such an example, a metal matrix composite is not desired, and the consumption of the carbon nanotubes is acceptable. One example of such an application includes carbothermic reduction of rare earth oxide materials to form rare earth metals. A carbothermic reduction of other metals to form pure metal without a composite dispersed phase is also contemplated. For example, refinement of copper oxide may be performed in oxygen, or merely with exposure to the oxygen in air. In such an example, the carbon nanotubes may be consumed, and refined copper will result without any significant dispersed phase.

In operation 206, the starting material is heated using microwave radiation. As described above, in other examples, other forms of wave energy radiation are utilized. In one example, a wave energy radiation is chosen to interact with components in the starting material such as the amount of carbon nanotubes to provide heat sufficient to drive a carbothermic reaction between the amount of carbon nanotubes and the amount of a metal oxide. In one example, a wave energy radiation is chosen to interact with components in the starting material such as the amount of carbon nanotubes to provide heat sufficient to melt the metal particles that form as a result of the carbothermic reaction.

Carbon nanotubes are particularly effective for use in the disclosed methods due to their high absorption of wave energy, in particular microwave energy, compared to other forms of carbon. In addition, carbon nanotubes have extremely high thermal conductivity compared to other forms of carbon. When a powdered starting material as described above is heated, the carbon nanotubes are especially effective at heating up quickly, and distributing the heat by conducting it along their lengths to different locations in the starting material. In examples where a carbothermic reaction is also desired, the carbon nanotubes provide a ready source of carbon, in addition to providing heat as a result of microwave absorption.

In operation 208, the metal oxide is reduced to form a metal matrix encapsulating at least a portion of the carbon nanotubes. In an alternate embodiment, the carbon nanotubes are partially or completely consumed, and the metal oxide is reduced to form a metal. In one example a metal oxide is selected with a low solubility for carbon. Metals with a low solubility for carbon may perform better as metal matrix materials with carbon nanotubes, as the dispersed carbon nanotube phase will be more stable in the metal matrix. Metals with a high solubility for carbon may absorb some or all of the dispersed carbon nanotube phase into the matrix during processing or after processing.

FIG. 3 shows x-ray diffraction scans of workpieces similar to workpiece 122 from FIG. 1 processed with exposure to oxygen in an air atmosphere. Diffraction peaks corresponding to copper oxide are shown as triangles in the figure, and diffraction peaks corresponding to metallic copper are shown as circles. As can be seen in the Figure after 5 seconds exposure to microwave radiation at 1.3 kilowatts of power, some metallic copper begins to appear in the workpiece. After 30 seconds exposure to microwave radiation at 1.3 kilowatts of power, the metallic copper peaks increase in magnitude, while the copper oxide peaks decrease in magnitude After 60 seconds exposure to microwave radiation at 1.3 kilowatts of power copper oxide peaks are absent, and metallic copper peaks have increased further in magnitude. The data in FIG. 3 confirms that the copper oxide portion of the starting materials is converting to metallic copper with application of heat from the microwave radiation.

FIG. 4 shows Raman spectra of a number of samples 410, 420, 430, 440, 450, and 460. Samples 410, 420, and 430 were processed with exposure to microwave radiation at 1.3 kilowatts of power in air. Sample 410 was processed in air for 60 seconds. Sample 420 was processed in air for 45 seconds. Sample 430 was processed in air for 30 seconds. Sample 440 was processed in argon for 45 seconds. Sample 450 was processed in argon for 30 seconds. Sample 460 indicates pristine carbon nanotubes as a baseline comparison.

A relative stability of carbon nanotubes incorporated into the reduced copper under argon compared to those incorporated in air is evidenced by R-values obtained from the Raman spectra shown in FIG. 4. R-values are defined as the ratio of the intensity of the D peak 402 at (˜1350/cm) to the G peak 404 at (˜1575/cm) and are indicative of the defect density within a carbon nanotube microstructure. FIG. 4 shows a Raman spectrum of pristine carbon nanotubes 460, as well as spectra taken of carbon nanotubes embedded in copper matrices during reactions in air (410, 420, 430) and argon (440, 450). Pristine carbon nanotubes 460 have an R-value of 0.95. Irradiation of carbon nanotubes for progressively longer times in both air and argon produced increasing R-values, though the carbon nanotubes heated in air had significantly higher defect concentrations for the same exposure time. Heating in air produced R-values of 1.08 and 1.11 after 30 and 45 s, respectively, compared with R-values of 0.99 and 1.06 for carbon nanotubes heated in argon for the same amounts of time. The copper-Cu₂O eutectic temperature has been reported as 1067° C., which is about twice the temperature needed to rapidly oxidize carbon nanotubes in air. Therefore, any carbon nanotubes subjected to such high temperatures in air are expected to have oxidized defect sites, and hence, high R-values.

FIG. 5 shows portions of a workpiece similar to workpiece 122 from FIG. 1 after various processing conditions. The sample in sub-figure a) shows a copper oxide portion 504 and a carbon nanotube portion 502 after exposure to microwave radiation at 1.3 kilowatts of power in argon for 15 seconds. The sample in FIG. 1 a) was not mixed prior to microwave heating. The copper oxide portion 504 is shown as a relatively large sintered mass with a thin surface layer 506 of reduced metallic copper on a surface of the copper oxide portion 504.

FIG. 5 b) shows a workpiece after exposure to microwave radiation at 1.3 kilowatts of power in argon for 15 seconds. The sample in FIG. 1 b) was mixed prior to microwave heating. A metallic copper particle 508 is shown, however the metallic copper is not uniformly distributed within the workpiece.

FIG. 5 c) shows an unmixed workpiece after exposure to microwave radiation at 1.3 kilowatts of power in argon for 45 seconds. FIG. 5 d) a mixed workpiece after exposure to microwave radiation at 1.3 kilowatts of power in argon for 45 seconds. FIGS. 5 c) and 5 d) are approximately the same scale. As can be seen, without mixing, large areas of consolidated metallic copper 510 are present with some remaining copper oxide 512. With mixing, however, as seen in FIG. 5 d) the resulting product is substantially uniformly mixed, and includes a good distribution of carbon nanotubes within a copper matrix.

As can be seen from FIGS. 4 and 5, in one example, mixed carbon nanotubes and metal oxide, when heated in an environment substantially isolated from external oxygen, provides a metal matrix composite material with good carbon nanotube integrity, and a substantially homogenous mixture of carbon nanotubes with a carbothermic reduced metal matrix.

EXAMPLES

Example methods of forming a metal matrix composite and selected properties of the resulting material are discussed below. Cu₂O (99% purity, 200 mesh powder) was purchased from Alfa Aesar (Ward Hill, Mass., USA). Commercially available NC7000 MWCNTs were obtained from Nanocyl (Nanocyl S.A., Sambreville, Belgium).

Metal samples were prepared using the following procedure. First, 0.40 g MWCNTs were placed into a high form alumina crucible. Then, 0.35 g Cu₂O was placed in the middle of the MWCNT bed, and the crucible was shaken lightly to sink the Cu₂O powder just below the surface of the MWCNTs. For some experiments, the powder was mixed with the MWCNTs using a spatula. The crucible was then placed in a domestic microwave (1.3 kW, Emerson, Parsippany, N.J., USA) and irradiated at high power for a set time. FIG. 1 shows a schematic depiction of the experimental set-up used in this study. For experiments conducted in an inert atmosphere, the glass chamber was flooded with argon for 1 min prior to microwave irradiation, and a steady gas flow was maintained until the crucible had cooled to room temperature.

The phases present during different stages of Cu₂O reduction in air were studied using XRD. To obtain fine particles for these powder diffraction experiments, samples were abraded with a steel file, and the filings were collected for analysis. A strong magnet was subsequently passed over the copper powder to ensure that no steel filings remained in the sample, and diffraction data were collected on a SCINTAG XDS2000 multi-purpose powder diffractometer at room temperature with Cu Kα radiation. Samples selected for optical, hardness, and SEM analyses were mounted in Bakelite, ground, and polished. The microstructures of partially reduced copper oxide and Cu-MWCNT composites were examined optically with an Olympus GX51 inverted metallurgical microscope and images were collected with an Olympus D12 digital camera system. The mechanical properties of Cu-MWCNT composites were evaluated using Vickers microhardness measurements, which were performed at 10 locations per sample with a LECO LM247AT microhardness tester applying a load of 100 gf over 13 s dwell time.

Micro-Raman spectroscopy was employed to determine the location and structural integrity of MWCNTs within the copper matrices of Cu-MWCNT MMCs. These measurements were collected using a Renishaw in Via spectrometer operating with a 488 nm Ar laser. SEM micrographs of Cu-MWCNT MMCs were acquired with an FEI Quanta-250 SEM operating at 10 kV under high vacuum, and EDS mapping was performed under the same conditions.

Cu₂O powder was reduced in both air and argon environments after different degrees of mixing of the oxide powder and MWCNTs prior to microwave irradiation. In the following discussion, “unmixed” refers to the experimental condition of Cu₂O placed in the middle of a bed of MWCNTs and “mixed” refers to the condition in which MWCNTs and Cu₂O were nearly homogeneously mixed prior to irradiation. To elucidate the reduction process of Cu₂O under these different conditions, reactions were run at short time intervals and the products were analyzed with optical microscopy and XRD. After 5 s of irradiation, relatively large pieces of porous, brittle Cu₂O with reduced copper shells were removed from the MWCNT bed. The large size of the particles indicates that the initially very fine Cu₂O particles sintered together quickly after microwave exposure. The reduction mainly occurred at the edges of the Cu₂O particles. This suggests that reduction occurred in the solid state, where slow diffusion would only allow the reaction to proceed in areas of intimate contact between oxide and carbon.

Close inspection revealed some small areas of copper in the interior of the large Cu₂O particle. These copper particles are likely the result of the oxidation of CNTs (and, correspondingly, the reduction of copper oxide) trapped between Cu₂O particles as they sintered together. After 10 s of irradiation, the particles still had porous Cu₂O cores, but the outer core of reduced copper was much larger and contained several dendrites of Cu₂O, indicative of melting. Furthermore, a eutectic structure of fine Cu₂O lamellae in a copper matrix was observed between the large dendrites. As the oxide and copper formed a melt, diffusion of carbon and oxygen were much faster than in the solid state, and the reduction reaction was accelerated.

Irradiation for 30 s resulted in particles with only small amounts of Cu₂O. Inspection revealed that these particles were composed primarily of copper dendrites with an interdendritic eutectic structure of Cu₂O and copper, This microstructure is again indicative of melting. Notably, several agglomerations of CNTs were also incorporated into the melt. Microwave exposure for 60 s resulted in spherical copper particles devoid of Cu₂O and containing small clusters of CNTs. Etching of samples irradiated for 60 s and subsequent micro-Raman measurements revealed that the majority of these CNT agglomerations were located on grain boundaries.

XRD was used to confirm the identity of the phases at different exposure times. The presence of small copper was revealed peaks after 5 s of irradiation that become much more intense as the irradiation time is increased. Conversely, the Cu₂O peaks became less intense and sharp with exposure time. The absence of CNT peaks in the XRD spectra of the 30 and 60 s irradiation time samples was noted, because relatively large agglomerations of CNTs were observed both optically and with micro-Raman spectroscopy. However, the XRD pattern of the NC7000 CNTs resulted in very weak and broad peaks that were difficult to distinguish.

In another example experiments conducted in air using mixed Cu₂O and MWCNTs. After a short exposure time of 10 s, very fine copper particles were observed to be evenly distributed in the MWCNT bed. The spherical nature of these particles, indicates that melting occurred. The absence of Cu₂O was also notable, but expected if melting occurred due to the high surface area of the droplet and rapid diffusion in the liquid state. Microwave irradiation for 30 s resulted in much larger spherical copper particles with a few agglomerated MWCNTs and no Cu₂O, as confirmed by micro-Raman and XRD measurements.

Copper particles formed after 60 s exposure time were even larger than those formed after 30 s and contained large MWCNT agglomerations, as shown in FIG. 4 c. These experiments, using mixed Cu₂O and MWNCTs in air, suggest that molten copper droplets are formed early on, which then wick together to form progressively larger spheres. MWCNTs trapped in the melt likely agglomerate due to van der Waals forces.

Experiments using unmixed and mixed Cu₂O were also performed in an argon environment. Similar to the case of unmixed Cu₂O in air, a large sintered mass of Cu₂O with a thin layer of surface reduction was formed after short exposure time. Further irradiation led to a reduction of the sintered Cu₂O, but at a much slower rate than in air. The slower rate likely resulted from the lower temperatures reached under argon due to the lack of oxidation of MWNCTs by ambient O₂, a highly exothermic event.

Irradiation of mixed Cu₂O for 15 s in argon resulted in much smaller pieces of reduced Cu₂O, analogous to the case of mixed Cu₂O in air. However, particles were not spherical, which indicates that melting did not occur. The large surface area of oxide in contact with carbon likely allowed for rapid reduction, even in the solid state. After 30 s of irradiation, brittle, porous structures composed of many sintered particles of fully reduced copper were obtained from the bottom of the crucible.

Further microwave exposure caused consolidation and partial melting of these structures. Because the reaction was run in an inert atmosphere, the absence of oxygen in the system inhibited extensive oxidation of the MWCNTs, as was the case when the reaction was carried out in air. As a result, samples produced under irradiation in argon contain many more MWNCTs than those irradiated in air.

The relative stability of MWCNTs incorporated into the reduced copper under argon compared to those incorporated in air is further evidenced by R-values obtained from micro-Raman spectroscopy. R-values are defined as the ratio of the intensity of the D peak (˜1350/cm) to the G peak (˜1575/cm) and are indicative of the defect density within a MWCNT structure.

As determined by Raman spectroscopy, pristine MWCNTs have an R-value of 0.95. Irradiation of MWCNTs for progressively longer times in both air and argon produced increasing R-values, though the MWCNTs heated in air had significantly higher defect concentrations for the same exposure time. Heating in air produced R-values of 1.08 and 1.11 after 30 and 45 s, respectively, compared with R-values of 0.99 and 1.06 for MWCNTs heated in argon for the same amounts of time. The copper-Cu₂O eutectic temperature has been reported as 1067° C., which is about twice the temperature needed to rapidly oxidize MWCNTs in air. Therefore, any MWCNTs subjected to such high temperatures in air are expected to have significant numbers of oxidized defect sites, and hence, high R-values. MWCNTs found in copper samples produced under argon have higher defect densities than pristine MWCNTs, which could be due to oxidation from oxygen dissolved in the molten copper or from residual atmospheric oxygen in the reaction chamber.

Microhardness measurements were used to determine the mechanical properties of the Cu-MWCNT composites prepared under different conditions. As a baseline, hardness measurements were taken of the pure copper shells found in samples microwaved for 5 s in air. Hardness tests were also performed on the MWCNT-copper composites made in the mixed condition and irradiation for 60 s in air and for 45 s in argon, and the results are summarized in Table 1.

TABLE 1 Vickers microhardness measurements. Exposure Average Standard Sample Atmosphere time [s] hardness [HV] deviation 1 Air 5 62.1 3.7 2 Air 60 72.5 4.6 3 Argon 45 121.0 6.1

The copper shells formed after 5 s of microwave exposure did not contain any MWCNTs, as reduction occurred in the solid state. The average hardness of these shells was 62.1 HV. The Hall-Petch relationship for the hardness of pure copper is given as:

H=13.40+16.50d ^(−1/2)

where H is Vickers hardness and d is grain size in mm. The average grain size of samples made by irradiation in air for 60 s was determined to be 98.1 mm by the linear intercept method. The hardness of pure copper with that grain size is predicted to be 66.1 HV by the Hall-Petch equation, though an average hardness of 72.5 HV was observed. Accounting for error in the hardness measurement, good agreement exists with the Hall-Petch relation, as is to be expected based on the small amount of MWCNTs incorporated into the copper. The grain size of the sample produced under argon in 45 s irradiation time was more difficult to determine. However, a markedly increased average hardness of 121.0 was observed. This high hardness value is likely caused by a combination of grain refinement and strengthening from MWCNT reinforcement.

Area fraction analysis using the images in FIGS. 6 a-6 d was performed to estimate the volume fraction of MWCNTs in 45 s argon composites, and an average value of 25.6 vol % was determined with a standard deviation of 2.9. This analysis treated all black areas in optical micrographs as 100% MWCNTs, so it gives an upper bound on the possible vol % MWCNTs. The actual figure may be lower due to the fact that the black regions contain some copper as well as small voids between individual MWCNTs.

FIG. 6 a represents a typical backscattered SEM image of a 45 s argon sample, and FIGS. 6 b and c are corresponding EDS elemental maps of copper and carbon. The copper and carbon elemental maps indicate that the black regions of the backscattered image are carbon rich, and that these regions, which contain MWCNT agglomerations, are uniformly distributed throughout the copper matrix. A representative secondary electron image of one of these MWCNT agglomerations is given in FIGS. 7 a and 7 b, in which individual MWCNTs can be seen bundled next to one another and surrounded by the copper matrix. The areas 602 in FIG. 6 d denote areas with strong carbon and copper signatures, and a secondary electron image of such a region is presented in FIG. 7 a. Arrows in this figure were added to demarcate individual MWCNTs emerging from the copper matrix.

EMBODIMENTS

To better illustrate the method and apparatuses disclosed herein, a non-limiting list of embodiments is provided here:

Embodiment 1 is a method that includes combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material, heating the starting material using wave energy radiation, and reducing the metal oxide to form a metal.

Embodiment 2 includes the method of embodiment 1, wherein combining an amount of a metal oxide comprises combining an amount of a rare earth oxide.

Embodiment 3 includes the method of any one of embodiments 1-2, wherein combining an amount of a rare earth oxide comprises combining an amount of a rare earth oxide chosen from a group consisting of Nd, Pr, La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y.

Embodiment 4 includes the method of any one of embodiments 1-3, wherein reducing the metal oxide to form a metal comprises reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.

Embodiment 5 includes the method of any one of embodiments 1-4, wherein the wave energy radiation is microwave radiation.

Embodiment 6 includes the method of any one of embodiments 1-5, wherein combining an amount of carbon nanotubes includes combining an amount of multi-walled carbon nanotubes.

Embodiment 7 includes the method of any one of embodiments 1-6, wherein combining an amount of a metal oxide includes combining an amount of copper oxide.

Embodiment 8 includes the method of any one of embodiments 1-7, wherein combining an amount of a metal oxide includes combining an amount of nickel oxide.

Embodiment 9 includes the method of any one of embodiments 1-8, wherein heating the starting material using microwave radiation includes heating for between about 30 seconds and 60 seconds.

Embodiment 10 includes the method of any one of embodiments 1-9, wherein heating the starting material using microwave radiation includes heating the starting material using microwave radiation having a frequency range between about 300 MHz and about 300 GHz.

Embodiment 11 is a method that includes combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material, substantially isolating the starting material from external oxygen, heating the starting material using microwave radiation, and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.

Embodiment 12 includes the method of embodiment 11, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in a gas.

Embodiment 13 includes the method of any one of embodiments 11-12, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in an inert gas.

Embodiment 14 includes the method of any one of embodiments 11-13, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in argon.

Embodiment 15 includes the method of any one of embodiments 11-14, wherein substantially isolating the starting material from external oxygen includes placing the starting material substantially in a vacuum.

Embodiment 16 includes the method of any one of embodiments 11-15, wherein combining an amount of carbon nanotubes includes combining an amount of multi-walled carbon nanotubes.

Embodiment 17 includes the method of any one of embodiments 11-16, wherein combining an amount of a metal oxide includes combining an amount of copper oxide.

Embodiment 18 is a method that includes combining an amount of carbon nanotubes with an amount of a powdered metal oxide, mixing the amount of carbon nanotubes with the amount of powdered metal oxide to form a substantially homogenous starting material, substantially isolating the starting material from external oxygen, heating the starting material using microwave radiation, and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.

Embodiment 19 includes the method of embodiment 18, wherein combining an amount of a powdered metal oxide includes combining an amount of powdered copper oxide.

Embodiment 20 includes the method of any one of embodiments 18-19, wherein combining an amount of a powdered metal oxide includes combining an amount of powdered copper oxide having particle sizes between about 150 mesh and 250 mesh.

Embodiment 21 includes the method of any one of embodiments 18-20, wherein combining an amount of carbon nanotubes with an amount of a powdered metal oxide includes combining a weight of carbon nanotubes with an about equal weight of powdered copper oxide.

Embodiment 22 includes the method of any one of embodiments 18-21, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in a gas.

Embodiment 23 includes the method of any one of embodiments 18-22, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in an inert gas.

Embodiment 24 includes the method of any one of embodiments 18-23, wherein substantially isolating the starting material from external oxygen includes enveloping the starting material in argon.

Embodiment 25 includes the method of any one of embodiments 18-24, wherein substantially isolating the starting material from external oxygen includes placing the starting material substantially in a vacuum.

Embodiment 26 includes the method of any one of embodiments 18-25, further including compacting the starting material prior to heating.

Embodiment 27 is a metal matrix composite that includes a reduced metal oxide matrix, and a carbon nanotube dispersed phase.

Embodiment 28 includes the metal matrix composite of embodiments 27, wherein the reduced metal oxide matrix comprises a reduced copper oxide matrix.

Embodiment 29 includes the metal matrix composite of any one of embodiments 27-28, wherein the carbon nanotube dispersed phase comprises multi-walled carbon nanotubes.

Embodiment 30 includes the metal matrix composite of any one of embodiments 27-29, wherein the metal matrix composite has a hardness in a range of between about 115 and 127 Vickers hardness.

The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.

In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

What is claimed is:
 1. A method, comprising: combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material; heating the starting material using wave energy radiation; and reducing the metal oxide to form a metal.
 2. The method of claim 1, wherein combining an amount of a metal oxide comprises combining an amount of a rare earth oxide.
 3. The method of claim 1, wherein combining an amount of a rare earth oxide comprises combining an amount of a rare earth oxide chosen from a group consisting of Nd, Pr, La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y.
 4. The method of claim 1, wherein reducing the metal oxide to form a metal comprises reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.
 5. The method of claim 1, wherein the wave energy radiation is microwave radiation.
 6. The method of claim 1, wherein combining an amount of carbon nanotubes comprises combining an amount of multi-walled carbon nanotubes.
 7. The method of claim 4, wherein combining an amount of a metal oxide comprises combining an amount of copper oxide.
 8. The method of claim 4, wherein combining an amount of a metal oxide comprises combining an amount of nickel oxide.
 9. The method of claim 4, wherein heating the starting material using microwave radiation comprises heating for between about 30 seconds and 60 seconds.
 10. The method of claim 4, wherein heating the starting material using microwave radiation comprises heating the starting material using microwave radiation having a frequency range between about 300 MHz and about 300 GHz.
 11. A method, comprising: combining an amount of carbon nanotubes with an amount of a metal oxide to form a starting material; substantially isolating the starting material from external oxygen; heating the starting material using microwave radiation; and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.
 12. The method of claim 11, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in a gas.
 13. The method of claim 12, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in an inert gas.
 14. The method of claim 13, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in argon.
 15. The method of claim 11, wherein substantially isolating the starting material from external oxygen comprises placing the starting material substantially in a vacuum.
 16. The method of claim 11, wherein combining an amount of carbon nanotubes comprises combining an amount of multi-walled carbon nanotubes.
 17. The method of claim 11, wherein combining an amount of a metal oxide comprises combining an amount of copper oxide.
 18. A method, comprising: combining an amount of carbon nanotubes with an amount of a powdered metal oxide; mixing the amount of carbon nanotubes with the amount of powdered metal oxide to form a substantially homogenous starting material; substantially isolating the starting material from external oxygen; heating the starting material using microwave radiation; and reducing the metal oxide to form a metal matrix encapsulating at least a portion of the carbon nanotubes.
 19. The method of claim 18, wherein combining an amount of a powdered metal oxide comprises combining an amount of powdered copper oxide.
 20. The method of claim 18, wherein combining an amount of a powdered metal oxide comprises combining an amount of powdered copper oxide having particle sizes between about 150 mesh and 250 mesh.
 21. The method of claim 18, wherein combining an amount of carbon nanotubes with an amount of a powdered metal oxide comprises combining a weight of carbon nanotubes with an about equal weight of powdered copper oxide.
 22. The method of claim 18, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in a gas.
 23. The method of claim 22, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in an inert gas.
 24. The method of claim 23, wherein substantially isolating the starting material from external oxygen comprises enveloping the starting material in argon.
 25. The method of claim 23, wherein substantially isolating the starting material from external oxygen comprises placing the starting material substantially in a vacuum.
 26. The method of claim 18, further including compacting the starting material prior to heating.
 27. A metal matrix composite, comprising: a reduced metal oxide matrix; and a carbon nanotube dispersed phase.
 28. The metal matrix composite of claim 27, wherein the reduced metal oxide matrix comprises a reduced copper oxide matrix.
 29. The metal matrix composite of claim 27, wherein the carbon nanotube dispersed phase comprises multi-walled carbon nanotubes.
 30. The metal matrix composite of claim 27, wherein the metal matrix composite has a hardness in a range of between about 115 and 127 Vickers hardness. 